The present invention relates to an electron gun for a cathode ray tube (CRT) and, more particularly, to an electron gun for a CRT that can produce uniformly-shaped electron beam spots over the entire screen area.
Generally in CRTs, the resolution of the display screen is largely dependent upon electron beam spot characteristics. To obtain a high-resolution display screen, electron beams should land over the overall screen area without any halo.
However, in the usual in-line type of electron guns having focusing and accelerating electrodes, electron beam spots may be horizontally distorted because cathodes for emitting thermal electrons to form red R, green G and blue B electron beams, and beam-passing holes of the electrodes are aligned in a horizontal line respectively.
Furthermore, non-uniform deflection magnetic fields generated by a deflection unit whose horizontal deflection field is pincushion-shaped and vertical deflection field is barrel-shaped are applied to the electron beams to deflect them in horizontal and vertical directions. These nonuniform deflection magnetic fields cause a focus defect called xe2x80x9castigmatismxe2x80x9d so that the resulting beam spots are liable to be distorted. As a result, the resolution of the display screen is seriously deteriorated.
Therefore, to solve such problems, a dynamic focusing electrode has been conventionally employed in the electrode system. In such a dynamic focusing electrode-based electron gun, the dynamic focusing electrode is applied with a relatively higher voltage than a static focus voltage so that the beam deflection deviation can be compensated for and electron beams deflected to land on the peripheral portion of the screen.
FIG. 11 is a cross sectional view of a prior art dynamic focusing electrode-based electron gun. The electron gun includes a triode portion composed of three cathodes 1 arranged in a horizontal plane, a control grid 3 and a screen grid 5. The cathodes 1, the control grid 3 and the screen grid 5 are sequentially disposed one after another to generate three electron beams of R, G and B. The electron gun further includes a static focusing electrode 7, a dynamic focusing electrode 9 and an accelerating electrode 11, that jointly constitute a main lens portion. The main lens portion accelerates and focuses the electron beam onto the screen. The static focusing electrode 7 and the dynamic focusing electrode 9 have lateral sides facing each other, where the lateral side of the former is formed with horizontally extended beam-guide holes 7a and the lateral side of the latter with vertically extended beam-guide holes 9a. 
In operation, a static focus voltage Vf is applied to the static focusing electrode 7, an anode voltage Ve, higher than the static focus voltage Vf, is applied to the final accelerating electrode, and a dynamic focus voltage Vd is applied to the dynamic focusing electrode 9. The dynamic focus voltage Vd is synchronized with a deflection signal of a deflection unit (not shown).
When the electron beam scanning operation is performed with respect to the central portion of the display screen, the dynamic focus voltage Vd is not applied to the dynamic focusing electrode 9 so that substantially circular-shaped beams land on that central screen portion.
In contrast, when the electron beam scanning operation is performed with respect to the peripheral portion of the display screen, the dynamic focus voltage Vd is applied to the dynamic focusing electrode 9. At this time, a 4-pole lens 13 is formed between the static focusing electrode 7 and the dynamic focusing electrode 9 to compensate for deviant deflection of the electron beams due to a lens 15 formed by the deflection unit. In this way, the deflected electron beams can land on that peripheral screen portion with a substantially circular-shaped spot.
However, when an abnormally high or low voltage happens to be applied to the dynamic focusing electrode, the aforementioned prior art electron gun is not equipped with a suitable means for remedying the abnormality, and hence the resulting abnormal dynamic lens cannot properly perform its function of compensating for distortion of the electron beams due to the non-uniform deflection magnetic fields. It naturally follows that the resulting beam spots are distorted.
It is an object of the present invention to provide an electron gun for a CRT that can make electron beams land over an entire screen area with a uniformly-shaped spot. This and other objects may be achieved by an electron gun having a triode portion composed of three cathodes arranged in a horizontal line and control and screen electrodes sequentially placed next to the cathodes. First to fourth focusing electrodes are sequentially arranged one after another next to the screen electrode. A final accelerating electrode placed next to the fourth focusing electrode.
The first focusing electrode has a first side facing the screen electrode and a second side facing the second focusing electrode. Three circular-shaped beam passage holes are formed in both sides of the first focusing electrode. The second focusing electrode has a first side facing the first focusing electrode and a second side facing the third focusing electrode. Three vertically elongated beam passage holes are formed in the first side facing the first focusing electrode and three circular-shaped beam passage holes are formed in the second side facing the third focusing electrode. The third focusing electrode has a first side facing the second focusing electrode and a second side facing the fourth focusing electrode. Three circular-shaped beam passage holes are formed in the first side facing the second focusing electrode and three vertically elongated beam passage holes are formed in the second side facing the fourth focusing electrode. The fourth focusing electrode has a first side with a common hole for allowing passage of the three electron beams.
A static voltage is applied to the first and third focusing electrodes, a dynamic voltage synchronized with a deflection signal is applied to the second and fourth focusing electrodes, and an anode voltage is applied to the final accelerating electrode.